Do not abort constructing TypeAlias if only type parameters hold us back.

mypy/semanal.py

@@ -5637,13 +5637,20 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
             else:
                 incomplete_target = has_placeholder(res)
 
-            incomplete_tv = any(has_placeholder(tv) for tv in alias_tvars)
-            if self.found_incomplete_ref(tag) or incomplete_target or incomplete_tv:
+            if self.found_incomplete_ref(tag) or incomplete_target:
                 # Since we have got here, we know this must be a type alias (incomplete refs
                 # may appear in nested positions), therefore use becomes_typeinfo=True.
                 self.mark_incomplete(s.name.name, s.value, becomes_typeinfo=True)
                 return
 
+            # Now go through all new variables and temporary replace all tvars that still
+            # refer to some placeholders. We defer the whole alias and will revisit it again,
+            # as well as all its dependents.
+            for i, tv in enumerate(alias_tvars):
+                if has_placeholder(tv):
+                    self.mark_incomplete(s.name.name, s.value, becomes_typeinfo=True)
+                    alias_tvars[i] = self._trivial_typevarlike_like(tv)
+
             self.add_type_alias_deps(depends_on)
             check_for_explicit_any(
                 res, self.options, self.is_typeshed_stub_file, self.msg, context=s
@@ -5677,7 +5684,10 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
             ):
                 updated = False
                 if isinstance(existing.node, TypeAlias):
-                    if existing.node.target != res:
+                    if (
+                        existing.node.target != res
+                        or existing.node.alias_tvars != alias_node.alias_tvars
+                    ):
                         # Copy expansion to the existing alias, this matches how we update base classes
                         # for a TypeInfo _in place_ if there are nested placeholders.
                         existing.node.target = res
@@ -5707,6 +5717,46 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
         finally:
             self.pop_type_args(s.type_args)
 
+    def _trivial_typevarlike_like(self, tv: TypeVarLikeType) -> TypeVarLikeType:
+        object_type = self.named_type("builtins.object")
+        if isinstance(tv, TypeVarType):
+            return TypeVarType(
+                tv.name,
+                tv.fullname,
+                tv.id,
+                values=[],
+                upper_bound=object_type,
+                default=AnyType(TypeOfAny.from_omitted_generics),
+                variance=tv.variance,
+                line=tv.line,
+                column=tv.column,
+            )
+        elif isinstance(tv, TypeVarTupleType):
+            tuple_type = self.named_type("builtins.tuple", [object_type])
+            return TypeVarTupleType(
+                tv.name,
+                tv.fullname,
+                tv.id,
+                upper_bound=tuple_type,
+                tuple_fallback=tuple_type,
+                default=AnyType(TypeOfAny.from_omitted_generics),
+                line=tv.line,
+                column=tv.column,
+            )
+        elif isinstance(tv, ParamSpecType):
+            return ParamSpecType(
+                tv.name,
+                tv.fullname,
+                tv.id,
+                flavor=tv.flavor,
+                upper_bound=object_type,
+                default=AnyType(TypeOfAny.from_omitted_generics),
+                line=tv.line,
+                column=tv.column,
+            )
+        else:
+            assert False, f"Unknown TypeVarLike: {tv!r}"
+
     #
     # Expressions
     #

mypy/semanal_typeargs.py

@@ -151,7 +151,7 @@ def validate_args(
 
         is_error = False
         is_invalid = False
-        for (i, arg), tvar in zip(enumerate(args), type_vars):
+        for arg, tvar in zip(args, type_vars):
             context = ctx if arg.line < 0 else arg
             if isinstance(tvar, TypeVarType):
                 if isinstance(arg, ParamSpecType):

test-data/unit/check-python312.test

@@ -2057,6 +2057,7 @@ reveal_type(AA.XX)  # N: Revealed type is "def () -> __main__.XX"
 y: B.Y
 reveal_type(y)  # N: Revealed type is "__main__.Y"
 [builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]
 
 [case testPEP695TypeAliasRecursiveInvalid]
 type X = X  # E: Cannot resolve name "X" (possible cyclic definition)
@@ -2168,3 +2169,26 @@ x: MyTuple[int, str]
 reveal_type(x[0])  # N: Revealed type is "Any"
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testPEP695TypeAliasRecursiveInParameterBound]
+from typing import Any
+
+type A1[T: B1] = list[int]
+type B1 = None | A1[B1]
+x1: A1[B1]
+y1: A1[int]  # E: Type argument "int" of "A1" must be a subtype of "B1"
+z1: A1[None]
+
+type A2[T: B2] = list[T]
+type B2 = None | A2[Any]
+x2: A2[B2]
+y2: A2[int]  # E: Type argument "int" of "A2" must be a subtype of "B2"
+z2: A2[None]
+
+type A3[T: B3] = list[T]
+type B3 = None | A3[B3]
+x3: A3[B3]
+y3: A3[int]  # E: Type argument "int" of "A3" must be a subtype of "B3"
+z3: A3[None]
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]